Cure monitoring and assessment of the cure completion is performed via extensive application of many technologies. Most conventional technologies require testing of witness coupons or samples cut out from the structure. Testing of witness coupons or actual samples of the material being cured is not real time and is an inaccurate means of assessing cure state of the actual structure material. Sampling methodology does not always result in satisfactory assessment of the curing conditions of the structure. For example, localized samples from larger polymer or composite structures often are not representative of the actual cure state of the material resulting in incomplete or unsatisfactory cure information.
In situ cure monitoring, among others, has been performed via various means, including optical, electrical, electromagnetic, mechanical and ultrasonic methods. Ultrasonic methods, for example, provide an advantage of directly sensing the mechanical modulus change of the curing material and thus directly monitor the structural cure of the matrix material. Widely demonstrated in a variety of curing applications, conventional ultrasonic measurements required to monitor and quantify the degree of cure can be cumbersome, expensive and requires complex set up and calibration procedures that make the application of this technology impractical for everyday use. Simple, real time and reproducible in situ cure monitoring is a significant problem facing many manufacturing applications. Current methodologies are inadequate for practical and economic ultrasonic cure monitoring needs for various cure applications, such as aerospace, civil, marine and related industries.
Several patents describe ultrasonic techniques for cure monitoring and some explore ultrasonic reflection for the potential cure monitoring. For example, U.S. Pat. Nos. 5,009,104 and 6,644,122 describe ultrasonic cure monitoring and evaluation of advanced materials and composites. Further, there is a group of ultrasonic cure patents that utilize time of flight (ultrasonic wave transit time) or signal loss (attenuation) measurement approaches (see e.g., U.S. Pat. Nos. 4,455,268; 4,515,545; 4,559,810; 5,911,159; 6,675,112) to monitoring materials modulus and cure state. Furthermore, embedded thin waveguide sensors explore cure effects on the waveguide walls and corresponding change to acoustical signal in and around different configuration waveguides (see e.g., U.S. Pat. Nos. 5,911,159; 4,904,080; 4,574,637; 4,590,803; and U.S. Patent Publication No. 2006/0123914). Some methodologies explore acoustical resonance (e.g., U.S. Pat. No. 4,758,803). Although physically correct, these approaches are entirely impractical and difficult to implement because of extensive and expensive tooling needs for multiple transducer, general loss of transducer after each process and the need to accurately measure transducer separation distances and ultrasonic wave travel times. U.S. Pat. No. 6,644,122 is directed to an ultrasonic cure monitoring process and describes a very general approach to cure monitoring; however this reference does not describe a sensing functionality or measurement process and simply states that the ultrasound responds to cure processes as a measurement tool. A majority of other patents, such as U.S. Pat. Nos. 7,245,371; 4,891,591 and 4,874,948 rely on other indirect sensing technologies that utilize non-mechanical, physically different means of estimating cure. However, none of the noted patents describe or teach a reproducible, differential and calibration approach of cure monitoring as in the embodiments of the present invention. Without the simplification of the test configurations, implementation of differential probes and implementation of calibration methods—the cure level measurements are arbitrary and have very limited engineering and applications value.
The present inventor's initial experiments utilizing ultrasonic cure using direct reflection coefficient (as reported in B. Boro Djordjevic “Cure Monitoring by Ultrasonic Reflection Coefficient”, Proc. SAMPE-ACCE-DOE, September. 27-28, Detroit, Mich. 1999 and B. Boro Djordjevic, B Milch “In-situ Ultrasonic Cure Monitoring Sensors” Proc. 43 Int'l SAMPE 98 Symposium, pp 967-967, May 31-July 4, Anaheim Calif. 1998) were successful in identifying an ultrasonic cure process, but impractical because of the unpredictable influence and variances in non-cure related signal variances, such as variances in temperature effects, variances in pressure effects, variances in transducer response, variances in waveguide response, and unspecified variances in instruments calibration effects that did not allow quantitative assessment of cure affected ultrasonic reflection signals and made impossible true comparison of the material cure level.
What is needed is a system and method that uses a direct, differential and calibrated approach for in situ monitoring of materials undergoing a cure process to ensure practical, reproducible, and comparable measurements of the cure process and degree of cure.